Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter (PM), nitrogen oxides (NOx), and unburned hydrocarbons (UHC).
Exhaust aftertreatment systems receive and treat exhaust gas generated by an internal combustion engine. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of regulated exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst. In some exhaust aftertreatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
The SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas. Generally, the SCR catalyst is configured to reduce NOx into constituents, such as N2 and H2O, in the presence of ammonia (NH3) and the catalytic materials of the SCR catalyst. Because ammonia is not a natural byproduct of the combustion process, it must be artificially introduced into the exhaust gas prior to the exhaust gas entering the SCR catalyst. Typically, ammonia is not directly injected into the exhaust gas due to safety considerations associated with the storage of gaseous ammonia. Accordingly, conventional systems are designed to inject a reductant (e.g., diesel exhaust fluid (DEF), ammonia, metal chloride salt, etc.) into the exhaust gas, which is capable of decomposing into gaseous ammonia in the presence of exhaust gas under certain conditions. The reductant commonly used by conventional exhaust aftertreatment systems is DEF, which is a urea-water solution.
A common SCR catalyst used in some parts of the world is a vanadium-based SCR catalyst. Generally, a vanadium-based SCR catalyst includes a carrier layer and a catalytic layer made from vanadium-based catalytic materials. Vanadium-based SCR catalysts provide certain advantages over SCR catalyst made from other catalytic materials. However, vanadium-based SCR catalysts are particularly susceptible to sulfur poisoning when sulfur is present in the exhaust gas stream passing through the SCR catalysts. Sulfur poisoning tends to reduce the efficiency of NOx reduction or conversion on the SCR catalyst.
For fuels with minimal sulfur content, sulfur poisoning of vanadium-based catalysts is avoided. However, for fuels with relatively high sulfur content, sulfur poisoning of vanadium-based catalysts is quite common. Accordingly, some engine systems include on-board diagnostics that detect the presence of sulfur poisoning, and derate the engine when sulfur poisoning is detected. Engines derated due to sulfur poisoning require servicing, which can be time consuming and expensive. Additionally, verification processes for verifying the presence of sulfur poisoning on a vanadium-based catalyst (e.g., to avoid false positives), or recovery processes for recovering a poisoned SCR catalyst, may be unavailable or ineffective. Therefore, vanadium-based catalysts initially diagnosed with sulfur poisoning commonly are replaced with new catalysts and discarded, which introduces high warranty costs and vanadium waste disposal problems. Alternatively, even if effective verification and recovery processes are available, such processes typically require excessive costs for operation and extensive personnel training.